This invention relates generally to flowmeters for metering liquids or gases, and more particularly to flowmeters of the vortex-shedding type and to improved obstacle assemblies and sensors therefor.
It is well known that under certain circumstances, the presence of an obstacle in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are known as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate. It is this phenomenon which is exploited to create a flowmeter. Flowmeters of this type are disclosed in the Bird U.S. Pat. No. 3,116,639, and in the White U.S. Pat. No. 3,650,152.
My prior U.S. Pat. No. 3,589,185 discloses an improved form of vortex-type flowmeter wherein the signal derived from the fluid oscillation is relatively strong and stable to afford a favorable signal-to-noise ratio insuring accurate flow-rate information over a broad range. In this meter, an obstacle assembly is mounted in the flow conduit, the assembly being constituted by a block positioned across the conduit with its longitudinal axis at right angles to the direction of fluid flow, a strip being similarly mounted across the conduit behind the block and being spaced therefrom to define a gap which serves to trap Karman vortices and to strengthen and stabilize the vortex street. This street is sensed to produce a signal whose frequency is proportional to flow rate.
In my copending application Ser. No. 354,803 there is disclosed an obstacle assembly in a vortex meter, the assembly being constituted by a fixed front section contoured to cause flow separation of the incoming fluid stream whose flow rate is to be measured, and a rear non-streamlined section which is shaped to interfere with the vortex street in the wake of the front section and is cantilevered from the front section to define a gap. The rear section is slightly deflectable relative to the front section whereby it is excited into minute vibrations by the vortex street. These vibrations are sensed by a strain gauge to produce a signal proportional to flow rate.
Because of the interfering effect of the rear section on the vortex street, one obtains a stronger vortex than with an ordinary obstacle assembly. The strength of the vortex determines the signal-to-noise ratio of the meter in that unless the vortex is strong, the signal is difficult to distinguish from turbulent noise produced by random flow disturbance that accompanies the regular oscillatory motion giving rise to the desired signal. As pointed out in said copending application, the vibratory motion of the rear assembly may be enhanced by securing a tail to the rear section, the tail extending downstream from the rear section. The use of a downstream tail is also disclosed in the above-identified White patent (FIG. 5), the tail taking the form of a semi-circular vane which is intended to stabilize the oscillatory flow in the downstream wake.
I have found however that the effectiveness of the tail depends on the phase relationship between the fluidic pressure forces produced in the gap between the front and rear sections of the obstacle assembly and the vortex forces produced behind the rear section, and that this phase relationship, by proper design of the tail, may be adjusted to optimize the effect of the tail.